Pour agave, simple syrup, or honey onto a shallow dish and place your glass rim-down in it. On another shallow plate, combine sugar and grapefruit zest, and dip the rim into the mixture to coat.
Combine tequila, grapefruit juice, simple syrup, and lime juice in a cocktail shaker half-filled with ice and shake well. Pour the mixture over your rimmed glass filled to the brim with ice.
Top with sparkling water and garnish with a slice of grapefruit and enjoy!